sba you have a bit of a mess there, would think that someone or ones have swaped and or alterd those 66's, but can't be sure, may be factory too!
You are goning to be very pleased with the sound when where done, as your cap values are probably all over the board...

My crossovers where bench tested and found to be within spec so after i resored the mid dome i was amazed at the quality of reproduction...now we are working on the speakers older design limitations...

I see a lot of interesting developments and new problems identified - more than I will have time to address in this session, so I'll start with some, do as much as I have time for, and will not forget the others, so don't worry, I'll get to everything eventually !

I'll post a few comments inside the Quote Box for specific clarity :-

Quote:

Originally posted by Grahame I'm back - sorry for the gap.

Alan - thanks for your very informative posts. I hope you haven't been affected by the recent flooding in the UK.

---> No direct flooding on me, but unfortunately was on various people .
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Interesting that Celestion seemed to go through so many crossover styles. Continuous improvement, or whatever they had in the parts bin /the supplier delivered- who knows?

---> Sometimes Manufacturers or Suppliers discontinue a part, then one has to buy another.
Also, when plastic film caps became readily available they were used as were both cheaper and promised longer service life than paper-in-oils, especially if got hot in use.
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One side effect of all this measurement is that I noticed that one of the MD-500s was cutting out intermittently - traced it down to the red/black wires on the outside must have been pinched at some point in all this opening / closing of the cases and would make / brake if you wiggled the wire . Solution - solder another wire in parallel from the connection on the back of the front plate to the tab at the base of the case - seems to have done the trick. Did it on both sides to be sure.

---> Unfortunately this is common, wire becomes weary when bent a lot, and it eventually breaks stand by strand.
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With all this assembling /disassembling are there any better solutions for semi - permanent connections that the rather flimsy seeming tab / folded spade arrangement ? Anything mil-spec designed for the task or better more modern solutions - or am I just being fussy? I could solder it in place + then just use the new form of connection where required.

---> Indeed "flimsy". You can, carefully, retension the folded spade with a small long-nosed pliers, but don't bend any part too sharply because that metal fatigues also, and eventually cracks and breaks off.

I'll give some thought to a more robust type of connection, and post about when decided.
This could be done when all the capacitors and resistors have been decided and installed, after which there may be no need for any more disassembly, thus all the wires could then be soldered directly to the board.
I'll keep thinking about this as there is a lot more to be done yet ...
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Ok, now onto DC Resistance - which is where it gets interesting ... or not.

LEFT Speaker
HF2000 (high) = 4.6
MD500 (mid) = 6.6
12"(low) = 4.2

RIGHT speaker
HF2000 (high) = 9.2
MD500 (mid) = 6.8
12"(low) = 4.2

Should I be concerned in the difference in R for the HF2000's ?
Any possible causes? any significance in the fact the values differ by a factor of 2?

To these cloth ears there doesn't appear to a gross difference -in fact they still sound fine to me - given that their placement is not ideal due to room / WAF constraints

I'm hoping to get some objective frequency response plots - so I can see if there is a difference, and what form it takes - if any.

---> Several matters here, I'll discuss below this quote box.

The 9.2 ohm HF2000 could be either :-

(A) an alternate model with twice the impedance voice-coil - such is common, and it may have been manufactured to use with a higher impedance mid or mid-bass driver in a different Celestion speaker, or, so that two tweeters could be used in Parallel for higher output treble level in a larger speaker, as those tweeters along with most tweeters of that era were fairly low power dissipation and became defective under the heat condition of high signal level drive {the adhesives would melt, and sometimes even the wire itself, and then if made of paper the coil formers would burn. Modern tweeters use high-temp capable formers and adhesives}.
Thus a sample of such tweeter could have got mixed up with a batch of the lower impedance ones and then installed - doesn't say much for Quality Control Inspections ! , unless a Service-man installed it later after the original tweeter became faulty, not realizing which impedance version was to be used.

(B) that tweeter is failing as result of its voice-coil rubbing against something, or similar mishap, which has worn away part of the diameter of the wire strand at one or more places - thinner wire has higher resistance.
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Is that tweeter still working ?

Play some music with a lot of Cymbals or Steel String Acoustic Guitar or Violins - quietly - and put your ear very close to the tweeter.
If it is working then listen to the other tweeter- at exactly the same ear distance - to hear if any any significant difference in output ... but, choose music where the same treble information is is both channels, ... or if you have an Audio Frequency Oscillater or a CD with Test Tones, then play some tones that are significantly above 5 kHz {so the mid-driver's output does not interfere}. Try between 8 kHz and 12.5 kHz. When the frequency gets to 16 kHz you may have difficulty hearing it, and as the frequency is swept higher - from about 6 kHz upwards, the degree you hear it depends very much on the angle your head is at towards the sound source. Try twisting your head - you may be surprised !

If no test tones, and you can't reliably hear the tweeter distinct from the mid-driver with music, then disconnect the mid-driver, BUT .. then you must only use low level signal, because when there is no terminating resistive impedance to that dual filter in the mid-range x-over there becomes 2 resonant points of almost short circuit {at two specific frequencies} - yes, even though you have not joined the 2 wires from the driver together.
There is only the two low DCRs of the two inductors as Load to your amplifier, thus use low volume output so as to not draw excess current from your amp which may damage your amp's output stage.
If you are worried, connect a resistor - a cheap 5 watt wire-wound is sufficient - of a resistance between 6.8 <--> 10 ohms where the MD500 would have been connected.

Post whether the 9.2 ohm HF2000 is working, and if at similar output level to the 4.6 ohm one.

I am VERY surprised that you are not hearing a difference in part of the treble spectrum, because the electrical effect of a double impedance tweeter connected to that treble filter will cause its cut-off to be at half the frequency, that is at 2.5 kHz instead of 5 kHz !
Thus there will be an octave of overlap between the MD500 and HF2000 in one of your 66s and not in the other.
No, that does not necessarily result in higher output between 2.5 k and 5 kHz, because in the 66 the two drivers are connected with one in the opposite Polarity to the other, thus there will be some cancellation of output as well as some addition - which where in that Octave, and in about another octave both below and above, will depend on the Phase relationships between the two drivers themselves as well as those caused by the mis-terminated treble filter.
But, all this should result in one 66 having significantly different Tone in part of its lower treble spectrum to the other 66 !

If the HF2000 is still working OK, then good, but don't play your 66 any louder than in the past, because that HF2000 being driven an octave below is putting a lot more power into it than the 66 design intended.
The extra power is still being dissipated in the tweeter even if it is being acoustically cancelled by phase differences, thus the tweeter can be damaged if driven harder.

I have two options for you to continue to use that HF2000 if it is stll working OK, thus post your assessment of it.
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The 25% higher DCR tweeter in #344 may simply be sample variation, or an accidentally overwound voice-coil, OR, as it seems your two Blackies were originally from different Production Batches the differences in the tweeters may have simply been two different Production Types - one for each period {such was an occasional event then}.

Also, swap the two Blackie cross-over boards - each to the other speaker.
Put the board with the two yellow 2.2uF Arcos into Blackie #249, because with the lower impedance tweeter there it will require larger capacitance to give close to the same crossover frequency that you will then get with the two 2uF Erie A304 caps put in circuit to the higher impedance tweeter in #344.
Yes both boards have the same summed equivalent of 6uF for the other treble filter cap, and the likely same value of inductance in the filter inductor, but with the boards swapped you will have closer to the same cross-over frequency than you currently have.

When you do that, have a look around the inductor coils, and at the under side of the board, to see if any Part Numbers for those inductors.
I am wondering if Celestion used different value inductance in the early model 66 than in the later model 66, at least in the mid-range filter, given that they have 30uF there instead of the 24uF used in the later models.
30uF there will significantly lower the cut-off frequency below the 500 Hz of the specification of the later model, thus the older MD500 will be working a lot harder as it has to move greater distance in-out to reproduce the lower frequencies sent to it via the 30uF cap, and it is a lower power rated MD500 !

Both pairs of 66's are in the same room...one pair on top of the other in a tower arrangement. The lower pair I'll call "blacky" because of the black front, and the upper pair I'll call "woody", for the wood front.

...and from a spare woofer: 4.2-4.3

Well, I read I mis-spelt your original "blacky" to blackie,
and your original "woody" to woodie, and now every-one is doing that, including yourself, thus I suppose you don't mind ... ?

Who are thes guys anyway - are they members of the Rolling Stones, or members of the Bay City Rollers ?
OK, so "Blackie" is the name of one of Eric Clapton's guitars, so perhaps Woodie or Woody is his cousin ? - well guitars are basically made from wood ...
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Is your spare woofer from the same production batch as one or other of the samples in your different 66s, or seeming later manufacture ?
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The "tower arrangement" - do you have the top of tower pair standing in the same orientation as the lower pair, or , do you have the upper pair upside-down ?

One orientation will give a different sound to the other, in both treble and bass response, and some other audible aspects.